Recent developments in thermal energy storage have focused on storing thermal energy in the form of a submerged ice mass in a water-flooded, insulated storage tank. As shown in U.S. Pat. No. 5,063,748, the ice mass may be formed by introducing ice particles at the bottom of the flooded tank to form an ice mass having the shape of an inverted cone. The tank includes a top structure for applying a counterbuoyant force to the ice mass to maintain the ice mass in a submerged state. The ice mass may be built continuously, or during off-peak electricity demand hours (usually, nights and weekends), by producing and delivering ice particles from a conventional ice plant adjacent to the tank. During those times when the thermal energy is needed to cool a load, usually during peak electricity demand hours, cold water is pumped from the tank to the load and thereafter returned to the tank at an elevated temperature, thereby melting ice and reducing the ice charge in the tank. Further details of such a thermal energy storage system are contained in U.S. Pat. No. 5,390,501 which describes a system for delivering ice from the ice plant to the thermal storage tank and U.S. Pat. No. 5,195,850 which describes a system for delivering the ice particles, against the buoyant force of the water in the tank, to the bottom of the tank.
In the operation of thermal energy storage tank systems as described above, it is important to maximize the thermodynamic efficiency of the system. One aspect of efficiency is the capability of consistently withdrawing the relatively colder water in the tank for delivery to the load. Various temperature gradients and eddy currents within the tank make consistent withdrawal of the coldest water a difficult task. Another issue is the desirability of maintaining the symmetry of the ice mass at all times, including times when the tank is almost fully charged with ice (e.g. a 95-100% ice charge) and times when most of the ice has been melted (e.g. a 5-10% ice charge). Symmetry of the ice mass results in more predictable operation of the system. More importantly, symmetry of the ice mass is critical for maintaining the stability of the ice mass and its associated counterbuoyant floating top. Stated differently, a significant loss of symmetry in the ice mass during numerous cycles of melting down the ice mass and subsequent rebuilding thereof can result in a loss of equilibrium and a roll over by the ice mass.
Thus, in the design and operation of thermal energy storage tank systems of the above-mentioned variety, there is a need for improved supply and return water loops and ice handling systems to enhance the thermodynamic efficiency of the system. Also there is a need to achieve the safety and operational advantages associated with retaining the essential symmetry of the ice mass during repetitive ice mass melting and rebuilding in an operational cycle.